The present invention relates to a process for the purification and recovery of propylene obtained from suspension polymerization plants for the manufacture of polypropylene, the suspension medium consisting mainly of propylene (the so-called mass polymerization).
The present invention moreover relates to the removal of atactic polypropylene formed during the polymerization, isotactic fines, catalyst residuals and additional suspension medium from the plant.
The polymerization of propylene in the liquid monomer, optionally in the presence of comonomers such as ethylene and higher .alpha.-olefins, is known. The reaction is carried out either in vessels provided with an agitator or in loop reactors. After completion of the polymerization a mixture containing solid polypropylene suspended in the excess liquid propylene and catalyst residuals is obtained. The liquid propylene further contains amorphous polypropylene and optionally low-molecular weight isotactic polypropylene in a dissolved state. A number of processes are known for working up this mixture.
For example, the suspension may be subjected to a flash evaporation for separating the monomer and the polymer, however, this process involves an absorption of the above-specified dissolved constituents on the product and a quality diminution of the latter. Furthermore, the vapors must be compressed again prior to being condensed in an economic manner (cf. British Pat. No. 10 40 669).
These disadvantages may be avoided by using a pressure decanter or a washing column, these processes yielding a powder with a relatively low content of atactic polymer and catalyst residual which is subjected to the further working up and drying.
The monomer obtained during the separation step must be worked up to remove the dissolved constituents prior to being recycled to the process.
It is further known to evaporate the propylene in a rectifying column at 40.degree. to 55.degree. C. (cf. German Offenlegungsschrift No. 15 20 569) or in a flash evaporator at 40.degree. to 70.degree. C. (cf. Japanese Patent Publication No. 52 005 881). However, agglomeration of the polymer, fouling of the heat exchanger tubes and of the column trays must be taken into consideration under these temperature conditions. Fouling likewise occurs in heat exchangers heated with high pressure steam.
A still further process is carried out above the critical point of propylene. In this case the atactic polypropylene is discharged in a liquid state after the phase separation. However, an unsatisfactory factor in this process is that the high reaction heat of the polymerization cannot be utilized, since it ranges at a low temperature level (cf. Japanese Patent Publication No. 75 020 046).